Electrical logging



Dec. 4, 14%. 5, M 2,390,409

ELECTRICAL LOGGING Filed Feb. 4, 1942 3 Sheets-Sheet 3 AMPLIFIER INDICATOR AL/C- J 82 INVENTOR:

COMP/{EH70}? B.

ATTO RN EY htented Dec. 4, i945 ELECTRICAL LOGGING Charles B. Aiken, North Plainiield, N. 3., assignor to Schlumberger Well Surveying Corporation, Houston, Tern, a corporation of Delaware Application February 4, 1942, Serial No. 429,476

Claims.

The present invention relates to the investigation of subterranean strata traversed by a bore hole and more particularly to a new and improved method and apparatus for obtaining indications which are related to variations in the dielectric constant of such strata at different depths in a bore hole.

Indications related to the dielectric constant of earth formations may be readily obtained by investigating the capacitive reactance of the formations to alternating current. At relatively low frequencies, that is, frequencies below about 200,000 cycles, the impedance of the soil is predominantly resistive, and relatively high frequencies must be employed in order to obtain impedance values having reactive components that are large enough to reflect variations in the dielectric constant of the formations. The use of frequencies high enough for this purpose, however, introduces a number of undesirable factors which make it diflicult to obtain accurate indications. For example, if the high frequency indications obtained in the bore hole are transmitted through relatively long conductors to indicating means at the surface of the earth. prohibitive errors may be introduced by variations in the impedance of those conductors and by other variable stray capacltances as the investigating apparatus traverses the bore hole.

It is an object of the present invention, accordingly, to provide a new and improved method and apparatus for obtaining indications of variations in the dielectric constant of earth formations traversed by a bore hole which is characterized by simplicity and accuracy of operation.

A further object of the invention is to provide a method and apparatus of the above character in which indicationsof variations in the dielectric constant of the formations are transmitted to the surface of the earth in the form of direct current values or alternating current values of low frequency.

A still further object of the invention is to provide a novel method and apparatus for obtaining indications of the phase difference between two electrical values, such as a current and a voltage, for example, which may be of variable magnitude.

A further object of the invention is to provide a novel method and apparatus of the above character in which an electrically energized electrode is disposed in a bore hole and indications ar obtained of the phase difference between the voltage between the electrode and a reference point and the current flowing through the electrode.

Another object of the invention is to provide a novel method and apparatus of the above charactor in which indications are obtained of the phase difference between a high frequency alterhating current passed through the formations and the potential difference between two points in the path of the current, which indications are transmitted to indicating means at the surface of the earth as direct current values or alternating current values of low frequency.

In accordance with the invention, high frequency alternating current is passed through the formations surrounding the bore hole and high frequency indications are obtained which are related to the dielectric constant of the formations. Specifically, high frequency indications are obtained either of variations in the impedance of the formations or of variations in the phase angle between the current flowing in the formations and the potential difference between two points in the current path. In either case, the high frequency indications obtained are converted either to direct current values or to alternating current values of low frequency, which may be transmitted with ease through relatively long conductors to suitable indicating means at thesurface of the earth.

Additional objects and advantages of the invention will appear from the following detailed description of several preferred embodiments, taken in conjunction with the accompanying drawings, in which:

Figure 1 is a schematic diagram of apparatus constructed according to the invention for studying the dielectric constant of subterranean formations traversed by a bore hole;

Figure 2 is a schematic diagram of one form -of electrical circuit for obtaining indications of the dielectric constant of the formations;

Figure 2A is a schematic diagram of a modiflcation of Figure 2 in which the sources of electrical energy are located at the surface of the earth;

Figure 3 is a schematic diagram of a modified circuit in which indications are obtained of the differences of phase between the current flowing through the formations and the potential difference between two points in the path of the current through the earth;

Figure 4 illustrates another circuit for obtaining indications of phase variations;

Figure 4A is a schematic diagram of a modification of Figure 4 in which the source of power for the apparatus in the bore hole is located at the surface of the earth; and

Figure 5 is a schematic diagram of apparatus including a power circuit and a separate potential circuit for obtaining indications of the dielectric constant of the surrounding formations.

Referring now to Figures 1 and 2, a pair of spaced apart electrodes l and H are shown disposed at a given depth in a. bore hole l2 containing a li uid l3. The electrodes l0 and H may be of any suitable size, shape and spacing and they are preferably mounted on an insulated, watertight casing I4 within which a source of electrical energy and part of the indicating circuit may be contained.

As shown in Figure 2, the electrodes l0 and II are connected through the conductors l5 and IE, respectively, to the apices I1 and I8, respectively, of a conventional type, alternating current, impedance bridge l9 mounted within the casing 4. The impedance bridge l9 comprises a plurality of fixed or adjustable impedances 20, 20a, 2| and 22 which are connected as shown in the drawings to form the other two apices 23 and 24 of the bridge, the electrodes l0 and I I and the intervening earth formations constituting one of the arms thereof. The values of the impedances 20, 20a, 2| and 22 should preferably be so chosen that the bridge [9 is balanced or nearly so when the impedance between the electrodes [0 and H is approximately the minimum formationimpedance encountered along the bore hole Connected across the apices 23 and I8 of the bridge I9 are two series connected sources of alternating current 25 and 26 which supply electrical energy for the energization of the bridge I9. The source 25 is adapted to generate alternating current of relatively low frequency, for example, about 400 cycles per second, and the source 26 is adapted to generate a relatively high frequency, such as, for example, 20 megacycles per second. To the apices I1 and 24 of the bridge is is connected an indicating circuit 21 includiing a pair of series connected filter networks 28 and 29 which are responsive only to alternating currents of 400 cycles and 20 megacycles, respectively. v

The filter network 28 may be designed in any conventional manner and it may include, for example, a condenser 30, an inductance 3i and a resistance 32 connected in parallel and tuned to parallel resonance at a frequency of 400 cycles. Similarly, the filter network 29 ma include a condenser 33, an inductance 34 and a resistance 35, also connected in parallel and tuned to parallel resonance at a frequency of 20 megacycles. The 400 cycle output of the filter network 28 is transmitted. through the conductors 36 and 31 to a conventional type detector or rectifier 38, and the output of the filter network 29 is transmitted through the conductors 37 and 39 to another conventional type detector or rectifier 40.

The output of the rectifier 38, which is proportional to the 400 cycle voltage output of the bridge I9, is transmitted through the conductors M and 42 in the supporting cable 43 to a suitable indicator 44, preferably of the recording type, located at the surface of the earth. In similar fashion, the output of therectifier 40 is transmitted through the conductors 45 and 42 in the supporting cable to a second indicator 46 located at the surface and also preferably of the recording type.

In operation, the casing M with the electrodes and II mounted thereon is moved through the bore hole 52 at any desired rate of speed while electrical energy is continuously supplied to the apices 23 and I8 of the bridge l9 from the 400 cycle and 20 megacycle sources 25 and 26. respectively, As the electrodes l0 and H pass formations of different electric-a1 impedances, the impedance of the arm of the bridge E9 in which they are included will vary and thus produce a.

corresponding unbalance of the bridge l9. Since the impedance of the filter 29 at 400 cycles is negligible, substantially all of the 400 cycle unbalance voltage, which is principally a function of the resistivity of the surrounding formations, will appear across the filter network 28. From thence it is transmitted through the conductors 33 and 31 to the rectifier 39 where it is converted into a direct current, the magnitude of which is proportional to the magnitude of the 400 cycle input to the detector or rectifier 38. The direct current output of the rectifier 38 is transmitted through the conductors 4| and 42 in the supporting cable 43 to the recording type indicator 44, thereby providing a continuous record of the resistivit of the formations surrounding the bore hole l2 as the casing I4 is moved therethrough. 7

Meanwhile, substantially all the 20 megacycle unbalance voltage, which is predominantly a function of the reactance of the surrounding earth formations, appears across the filter network 29 because its impedance at 20 megacycles is very high whereas the impedance of the filter 28 at that frequency is negligible. This 20 megacycle voltage is transmitted through the conductors 31 and 39 to the detector or rectifier 40 where it is converted into a direct current, the magnitude of which is a function of the 20 megacycle voltage input to the rectifier 40. The direct current output of the rectifier 40 is transmitted through the conductors 45 and 42 in the supporting cable 43 to the recording type indicator 46 at the surface of the earth where it provides a continuous record of variations in the impedance of the earth formations traversed by the bore hole.

The relative contributions of soil resistance and reactance to the total impedance will depend upon the type of formation and upon the frequency used. At 20 megacycles, the impedance of many formations will be predominantly reactive, although some may show effects of resistance. In any event, the impedance curve obtained will differ radically from that obtained at low frequencies and will be a valuable guide to the variations in dielectric constant of the formations. By employing a lower frequency, the relative importance of the capacity reactance of the soil will be reduced, while if a high frequency is used it will be increased.

It will be observed that the apparatus described in detail and illustrated in Figure 2 of the drawings provides simultaneously records of variations in the resistivity and in the dielectric constant of the earth formations as the casing I4 and the electrodes l0 and H traverse the bore hole [2.

If desired, the 20 megacycle voltage supplied by the source 26 might be modulated with a low frequency voltage, such as, for example, about 250 cycles per second, in which case the output of the detector or rectifier 40 would contain a 250 cycle component of voltage, variations in the magnitude of which would be proportional in magnitude to variations in the dielectric constant of the formations. Where this is done, the recording type indicator 46 should be designed to provide indications of 250 cycle alternating current values, only. The rectifier 38 may also be omitted, in which case the 400 cycle output of the bridge I9 could be transmitted to the surface of the earth through the same conductors in the cable 43 as are used for the 250 cycle voltage. This alternative requires that the recording type indicators 44 and 46 be responsive only to 400 and 250 cycle alternating currents, respectively. The source 25 might also be a source of direct current, in which event the indicator 44 shouldbe responsive only to direct current values. When the source 25 is direct current, a direct current bridge unbalance appears between apices l1 and 24, with the result that part of the unbalance appears between l1 and the midpoint between the filters 2B and 29. The proportion of direct current unbalance voltage that appears across the filter 28 and,

therefore, between the conductors 4! and 42, de-

pends only upon the ratio of the resistances 35 and 32 in the two filter circuits. The 20 megacycle signal will have superimposed upon it the amount of the direct current signal that appears across the resistance 35. This signal is very small and the direct current supplied to the rectifier 40 will be too small to have any significance.

As a further modification, the sources 25 and 28 might be located at the surface of the earth and alternating current supplied therefrom to the bridge I 9 in the bore hole l2 through suitably shielded electrical transmission lines, as illustrated in Figure 2a. In that figure, the sources 25 and 28 are shown connected to the conductors 42 and 45 in the supporting cable 43. In order to prevent the flow of direct current from the conductors 42 and 45 to the sources 25 and 26, it is desirable to insert a condenser 87 in series therewith as shown in the figure, It is also desirable to insert conventional type low pass filters 88 and 39'between the conductors 4i and 42 and the indicating instrument 44 and between the conductors 42 and 45 and the indicating instrument 44, respectively, for the purpose of preventing the passage of alternating current from the sources 25 and 25 to the indicating instruments 44 and 45.

The 20 megacycle alternating current from the source 25 is transmitted through the conductors 42 and 45 and the conductors 90 and St to the input terminals of a conventional type amplifier 92, which is provided with automatic volume control so that its output will remain constant regardless of variations in the magnitude of the input thereto. A condenser 803 may be inserted in series with the conductor 5| to block the passage of direct current and low-frequency alternating current to the amplifier 32. The output of the amplifier 92 is transmitted through the conductors 93 and 94 to the primary winding 95 of a transformer 96, the secondary winding 97 of which is connected to the apices 23 and I8 of the bridge l9,

The 400 cycle alternating current from the source 25 is transmitted through the conductors 42 and 45 and the conductors 98 and 99 to the primary winding I 00 of a, transformer IOI, the secondary winding I02 of which is connected in series with the secondary winding 91 of the transformer 96. A choke coil I04 may be inserted in series with the conductor 38 for blocking the passage of 20 megacycle alternating current to the transformer ml.

By virtue of the automatic volume control feature of the-amplifier 92, the 20 megacycle voltage supplied to the bridge 19 will remain substantially constant in magnitude regardless of any material attenuation that may occur in transmission through the relatively long supporting cable 43. The attenuation of the 400 cycle alternating current in its transmission through the supporting cable 43 will be relatively small and consequently an automatic volume control amplifier is not required, although one could be used, if desired.

In order to prevent the power voltages from the sources 25 and 28 from affecting the rectifiers 38 and 40, conventional type low-pass filters I05 and I06, respectively, may be inserted between the rectifier 38 and the conductors 4| and 42 and between the rectifier 40 and the conductors 42 and 45, as shown in Figure 2a. The filters I05 and I08 are shown as conventional one-section filters, but additional sections may be used, if desired. Even with this embodiment there will be a. limit to thedepth of holejhatflcan be surveyed with a high frequency source of power located at the surface of the earth because of excessive attenuation produced in the cable 43 at great depths.

In the modification illustrated in Figure 3 of the drawings, indications re obtained of the difference in phase between an alternating current transmitted through the earth formations surrounding the bore hole, and the potential difference between two points in the current path, which difference in phase is known to be a function of the dielectric constant of the formations.

The apparatus for accomplishing this result includes a source of alternating current of high frequency 26, such as, for example, 20 megacycles, which is located within the casing 14 and which is connected through the conductor 41 to the electrode l0 and through the conductor 48 and a fixed resistance 49 to the electrode H. The resistance 49 should be substantially non-inductive and non-capacitive so that the potential drop across it will be in phase with the current flowing therethrough. The potential difference across the resistance 49 is supplied through the conductors 50 and 5! to the input terminals of a conventional type amplifier 52 having automatic volume con trol. The function of this amplifier is to provide an output which is substantially constant in magnitude regardless of variations in the input voltage and the phase of which is substantially identical with, or bears a fixed relation to, the phase of the voltage input.

The potential difference produced between the electrodes in and II by the passage of the alternating current through the earth formations surrounding the bore hole [2 is supplied through the conductors 53 and 54 to the input terminals of a second conventional type amplifier 55 which is also provided with automatic volume control so that its output is substantially constant in ma'gnitude and substantially identical in phase with the input voltage. The ground output terminals of the amplifiers 52 and 55 are connected together through a conductor 56 and the other output terminals of the amplifiers 52 and 55 are connected together through a pair' of series connected resistors 51 and 58, the common point 59 between the resistors 51 and 58 being connected to the ground conductor 55 through a conductor 60. It is not essential that the phase of each amplifier output voltage be the same as that of its input voltage, provided any phase shift between input and output is the same for both amplifiers.

It will be apparent from the connections described above and illustrated in Figure 3 that if the output voltages of the amplifiers 52 and 55 are the same in magnitude and out of phase, the potential difference across the two series connected resistors 51 and 58 will be zero. Morethrough the conductors GI and 62 to a conventional type rectifier 62a, the direct current output of which is transmitted through the conductors 42 and 45 in the cable 43 to the recording type indicator 46 at the surface of the earth. As the casing I4 and the electrodes I0 and II traversethe bore hole l2, the recording type indicator 45 will provide a continuous record which will be related to variations in the dielectric constant of the formations in the vicinity of the electrodes I0 and II.

An alternative form of apparatus for comparing the phase of the current transmitted through the formations with the phase of the potential difierence between the electrodes I0 and II is illustrated in Figure 4. Referring to Figure 4, one terminal of the source 26 in the casing I4 is connected through the conductor 63 to the electrode l0 and to the ground output and input terminals of a conventional type amplifier 64 provided with automatic volume control. The other terminal of the source 25 is connected through a conductor 85a, a resistance 65 and the conductor 61 to the ungrounded input terminal of the amplifier 54 and to the electrode II, thereby establishing a 20 megacycle alternating current field in the intervening earth formations between the electrodes I0 and H.

The output voltage of the highfrequency source 26 is also supplied to a conventional type pentode vacuum tube 68, the cathode 69 of which is connected to the conductor 63 and the plate I0 of which is connected through a condenser II to the conductor55a. The potential difference betwen the electrodes I 0 and II, the phase angle of which is a function of the dielectric constant of the adjacent earth formations, is-impressed upon the input terminals of the amplifier 64. The output of the amplifier 54, which is held substantiall constant in magnitude and is in phase with, or bears a fixed phase relation to, the input voltage, is supplied to a conventional type phase adjusting means 12 which serves to adjust the phase of the output of the amplifier 64. One outthe potential applied between the cathode 69 and the plate I0 of the pentode 68, the latter is adjusted to pass no plate current. This is accomplished by operating the phase adjusting device 12 and the variable bias I6 until the control grid I1 of the tube 68 goes negative when the plate 10 goes positive, the magnitude of the grid voltage being just sufiicient to prevent the fiow of plate current. When the alternating voltage applied to the grid has a positive sign, the plate voltage will be negative and current will still fail to flow. However, if thephase of the voltage delivered by the amplifier 64 changes, because of changes in phase of the voltage across the electrodes I0 and I I, the grid 'I'I will not be at its of the pentode 68 is transmitted through a re-,

sistance 18, connected in parallel with the condenser II. The potential drop across the resistance I8 is transmitted through the conductors 42 and 45 in the supporting cable 43 to the recording type indicator 45 at the surface of the earth. In this fashion, a continuous record is provided of a function of variations in the dielectric constant of the earth formations in the vicinity of the electrodes I0 and II. If desired, a pair of choke coils 18a and 1827 may be placed in series with the conductors 42 and 45,- respectively, to prevent alternating current from passing therethrough. These are not essential, however. Other suitable tubes may also be used in place of the pentode 68. i

The power for operating the equipment in the investigating apparatus l4 can, if desired, be

. the conductors I08 and I09 to the primary windtransmitted from the surface as an alternating current of low frequency substantially difierent from any of the frequencies used in the measuring operations, as shown in Figure 4a. Referring to this figure, a source of alternating current II4 of 700 cycles frequency, for example, is connected to the conductors 42 and 45. A conventional type filter I01 may be inserted between the conductors 42 and 45 and the direct current indicating instrument 45 for the purpose of blocking the passage of alternating current to the latter. At the lower end of the cable 43 in the bore hole, the alternating current is transmitted through the conductors 42 and 45 and ing IIO of a transformer III, the secondary winding II 2 of which is connected to a conventional full-wave rectifier II3 of any suitable type.

The output of the rectifier II 3 is filtered by a conventional type inductance-capacity filter H5. Direct current voltage for the operation of the oscillator 26a is supplied from the terminals of the filter II 6 through the conductors II! and I I8. Voltage for the operation of the screen grid I20 of the pentode 68 is supplied through a resistance H9, and a by-pass condenser I200 is connected between the screen grid I20 and ground.

Grid biasing voltage for the pentode 58 is provided by means of a variable resistor I22, the variable contact I23 of which is connected to the cathode 6901? the pentode B8 and the terminals of which are connected through a resistance I24 and the resistance H9 to the terminals of the filter H0. The heaters of the pentode. and the vacuum tubes in the oscillator 26a may be energized by 700 cycle alternating current obtained from a secondary winding I21 on the transformer I I I.

' Except for the differences noted above, the

circuit shown in Figure 4a is identical with the circuit shown in Figure 4 and it operates essentially in the same manner insofar as the well logging operations are concerned.

In the embodiment illustrated in Figure 5 of the drawings. the high frequency source is located at the surface of the earth, its terminals being connected through the conductors 4| and 42 in the supporting cable 43 to the input terminals of a conventional amplifier I25 having automatic volume control. One output terminal of the amplifier I25 is connected to a ground electrode H6 in the drill hole which is preferably some distance from electrodes 8|, l0 and II. The remaining output terminal of the amplifier I25 is connected to a fixed, substantially noninductive and non-capacitive resistance 80 which in turn is connected to an electrode 8| disposed inthe bore hole. Because of th automatic control feature of the amplifier I25, this circuit sets up a substantially constant electric field in the earth formations surrounding the bore hole.

Directly below the electrode BI and spaced apart therefrom are located the two electrodes ill and H which are connected through the conductors 82 and 83, respectively, to the input terminals of a phase comparing device 84 located within the casing M, which may be of the type shown in either of Figures 3 and 4 and described hereinabove. The potential difference across the resistance 80, which is in phase with the current flowing to the electrode 8|, is supplied through the conductors 85 and 86 to the phase comparing device 86. The direct current or low frequency alternating current output of the device 84 is transmitted through the conductors 42 and 45 indications to be obtained of variations in the I dielectric constant of earth formations traversed by a bore hole more accurately and with greater simplicity than has been possible heretofore. By

virtue of the fact that the high frequency indications obtained in the bore hole are transmitted to the surface of the earth as direct current values or alternating current values of low frequency, the difiiculties encountered heretofore in transmitting the high frequency indications obtained to the surface of the earth have been entirely eliminated.

The several embodiments of the invention described above are intended merely to be illustrative and not restrictive and they are susceptible of numerous changes in form and detail within the scope of the appended claims.

I claim:

1, Apparatus for investigating earth formations traversed by a bore hole, comprising a pair of spaced apart electrodes in the bore hole, an electrical network in the bore hole and connected to said electrodes to form an impedance bridge tric constants and resistivitles, respectively, of the earth formations traversed by the bore hole, means in the bore hole for converting the high and low frequency outputs of the bridge to electrical values that can be transmitted through relatively long conductors without material change, electrical indicating means at the surface of the earth, and circuits for conducting said electrical values to different indicating means.

2. Apparatus for investigating earth formations traversed by a bore hole, comprising a pair of spaced apart electrodes in the bore hole, an electrical network in the bore hole and connected to said electrodes to form an impedance bridge of which the electrodes and the intervening formations constitute one arm, a source of high frequency alternating current for the bridge, a source of low frequency alternating current for the bridge, said high frequency and low fre quency sources being adapted to energize said bridge simultaneously, means for separating the high and low frequency outputs of the bridge, means for rectifying said high and low frequency outputs of the bridge to simultaneously provide respective component electrical values that can be transmitted through conductors of considerable length without material change, indicating means at the surface of the earth, and electrical means for transmitting said respective component electrical values to said indicating means.

3. Apparatus for investigating earth formations traversed by a bore hole, comprising electrical investigating means in the bore hole, said investigating means being adapted to be energized by alternating currents of high and low frequencies to provide electrical values of high and low frequenciesthat are functions of electrical characteristics of the surrounding formations, high and low frequency sources of alternating current at the surface of the earth, an energizing circuit extending from said sources to said investigating means in the bore hole, electrical means connected to said energizin circuit for supplying to said investigating means a high frequency alternating voltage of substantially constant magnitude, second electrical means connected to said energizing circuit for simultaneously supplying low frequency alternating current to said investigating means, separate electrical means for rectifying the respective high and low frequency electrical values provided by the investigating means, indicating means,located at the surface of the earth, and electrical connections between said rectifying means and indicating means.

4. Apparatus for investigating earth formations traversed by a bore hole, comprising a pair of spaced apart electrodes in the bore hole, an electrical network in the bore hole and connected to said electrodes to form an impedance bridge of which the electrodes and the intervening formation constitute one arm, high and low frequency sources of alternating current at the surface of the earth, an electrical circuit connected to said sources and said impedance bridge for supplying high and low frequency altemating current to the energizing diagonal of the latter, a plurality of filter means connected to the output diagonal of said bridge for separating the high and low frequency outputs of the bridge, a plurality of rectifying means connected to said respective filter means, indicating means at the surface of the earth, and electrical circuit means including said first named electrical circuit for transmitting the outputs of said rectifying means to said indicating means. A

5. Apparatus for investigating earth formations traversed by a bore hole; comprising a pair of'spacedapart electrodes in the bore hole, an electrical network in the bore "hole and connected to said electrodes to form an impedance bridge 01' which the electrodes and the intervening formations constitute one arm, high and low frequency sources of alternating current at the surface oi. the earth, an energizing circuit connected to. said sources and extending to said pedance impedance bridge in the bore hole, an automatic volume control amplifier connected to bridge. a plurality of filter means for separating the high and low frequency outputs of the bridge, a plurality of rectifying means connected to said respective filter means; indicating means at the surface 01' the earth, and electrical circuit means including said energizing circuit connected to said indicating means and rectifying means.

CHARLES B. AIKEN. 

